As is known in the art, there is a requirement in many applications for the active cancellation of large signal interferers in radio frequency (RF) and microwave receivers. Numerous active cancellation schemes based on feed-forward (see for example: S. Ayazian, and R. Gharpurey, “Feedforward interference cancellation in radio receiver front-ends,” IEEE Transactions on Circuits and Systems-II Express Briefs, vol. 54, no. 10, pp. 902-906, October 2007 and H. Darabi, “A blocker filtering technique for SAW-less wireless receivers,” IEEE Journal of Solid State Circuits, vol. 42, no. 12, pp. 2766-2773, December 2007) or feedback (see for example T. Werth, C. Schmits, R. Wunderlich, and S Heinen, “An active feedback interference cancellation technique for blocker filtering in RF receiver front-ends,” IEEE Journal of Solid-State Circuits, vol. 45, no 5, pp, 989-997, May 2010) approaches have been developed, but incorporate a low-noise amplifier (or gain amplifier) in the primary signal path as a means to achieve cancellation. The incorporation of this active component in the primary signal path limits the power handling capabilities of the active cancellation circuitry. Similarly, many approaches only provide active cancellation for continuous wave (CW) signals or modulated/encoded signals based on relatively slow (<1 Mbps) modulation/encoding schemes, and require a finite amount of time for the cancellation to synchronize with the system. The ability to have the cancellation error signal generated and utilized in the cancellation approach within the first period of the interfering signal is highly desirable, as well as the ability to cancel moderate power (>10 dBm) interferers without the use of high-linearity (and high power) LNA's in the primary path.
As is also known in the art, feed-forward active cancellation is based on the ability to generate an error signal that is identical in amplitude and 180 degrees out of phase with the interfering signal, and then combine this error signal with the interfering signal to cancel it out. An approach previously developed is shown in FIG. 1. Here, the input signal (e.g., RF/microwave frequency having both the desired signal and the interfering signal of known radio frequency) is sampled and fed to an auxiliary path. The desired signal and the interfering signal in the auxiliary path are down-converted in frequency with in-phase (I) and quadrature (Q) local oscillator signals (LOI, LOQ) and a pair of mixers, as shown; with the interfering signal being converted to a known intermediate frequency (IF) or baseband frequency. The frequency down-converted signal is then amplified (or attenuated) to achieve the correct amplitude for maximum cancellation. The signal is also sent through a bandpass or lowpass filter, which is tuned to the known IF or baseband frequency, to filter out all other signals (i.e. the desired signal) leaving only the interferer/error signal in the auxiliary path. This remaining interfering signal is then up-converted in frequency by in-phase (I) and quadrature (Q) local oscillator signals (LOI, LOQ) and a pair of mixers, as shown, back to the RF/microwave frequency of interest (i.e., the original known interfering radio frequency) and combined with the full spectrum of the signal in the primary path (i.e., the input signal) to enable cancellation of the interfering signal in the primary path. As shown in FIG. 1, the auxiliary path performs both the frequency down-conversion and up-conversion in a pseudo-weaver architecture (B. Razavi, “RF microelectronics”, Upper Saddle River, Prentice Hall, 1998), to address image rejection concerns during up-conversion. Amplitude alignment and phase alignment of the interfering signal in the primary and auxiliary paths must also be addressed to achieve cancellation, either by minimizing the phase and amplitude variation in the two paths, or by compensating for the variation in the primary path or in the auxiliary path.
Although the system described above in connection with FIG. 1 has demonstrated reasonable performance in previous works noted above, its usefulness in broader applications has been limited by 3 critical factors:
1) Accurate phase alignment of signals
2) The ability to handle large interfering signals
3) The ability to provide near instantaneous protection to circuitry that appears later in the receive chain (i.e. a very brief start up time for signal alignment).
In order for active cancellation circuitry to achieve high levels of cancellation (>30 dB) the error signal being generated must be within 2 degrees of phase and 0.5 dB of amplitude to the interfering signal. Although high levels of amplitude control are possible in most monolithic technologies, high levels are phase control are less readily available. Even in monolithic silicon approaches, which provide high levels integration and minimal signal delay between components, the phase delay associated with individual building blocks create challenges for phase alignment in these feed-forward cancellation schemes. These phase and amplitude alignment challenges limit the bandwidth of operation for these feed-forward approaches, as well as the magnitude of cancellation that can be achieved.
Another challenge faced with many feed-forward cancellation approaches, is the ability to maintain operation in the linear region while addressing the interfering signal. As shown in FIG. 1, the low noise amplifier (LNA) in the primary path must stay in the linear region of operation even with the presence of the interfering signal. This requirement forces an upper limit on the input power received by the active cancellation system, and can require very high linearity (and typically high power dissipation) circuitry to be incorporated for the LNA and up-conversion mixers to maximize the dynamic range of the system.
Lastly, the approach shown in FIG. 1 requires a finite amount of start-up time, before the error signal is created and can be applied to the interferer to cancel it out. This start up time is roughly the inverse of the bandwidth of the bandpass filter used in the auxiliary path. During this finite start-up time, the interfering signal will be amplified and sent into the receiver, possibly saturating or even damaging components further down-stream in the receiver.